Wafer inspection systems are often used to analyze wafers (or “dies”) in order to determine the presence of potential defects. A typical wafer inspection system will generate an image of the die to be analyzed and compare this image to a reference image, which may be taken from a database or the image of another die in the series. Spot scanning architectures generate an image of a wafer pixel by pixel by scanning a focused beam of illumination across the sample and detecting light scattered and/or reflected from the sample. In this way, spot scanning systems are capable of detecting features on a wafer with high spatial resolution.
In general, oblique spot scanning wafer inspection systems are configured such that the illumination beam interacts with the wafer at an oblique angle rather than at a normal incidence angle. In the field of wafer inspection systems, oblique angle incidence enables the detection of polarization-induced effects on the sample. Additionally, many surface features such as integrated circuits approximate a diffraction grating; the use of an oblique sampling beam thus enables precise diffraction-based monitoring of wafer features. However, the use of an oblique angle sampling beam in traditional wafer inspection systems may reduce the throughput, or alternatively the efficiency, of a wafer inspection system. This is because only a single linear region may be sampled at a given time. This linear region is described by the intersection of the focal plane of the objective lens, which is typically normal to the optical axis of the objective lens, and the plane of the wafer. A typical oblique angle scanning wafer inspection system will scan an illumination beam along this linear region and detect scattered and reflected light from the sample with one or more detectors. A two-dimensional image is generated through the acquisition of successive line scans in which the sample is moved via a translation stage between successive line scans. Therefore, there exists a critical need to develop systems and methods to increase the throughput of oblique scanning wafer inspection systems.